It takes seconds to install a Firewire card.
* switch your computer off and unplug it
* take the case off your computer
* make sure you've got a spare PCI slot
* you may need to take the blanking plate off the case so that the firewire sockets can poke through
* push the firewire card firmly into the slot (don't touch any of the electronics as you may accidently damage them)
* put the case back on
* plug your computer back in and switch your computer back on
Everything should be OK.
And just think, if you went to PC World, they would have charged you 30 odd quid! All donations welcomed!